The Hamburg/ESO R-process enhanced star survey (HERES)^*,**

IV. Detailed abundance analysis and age dating of the strongly r-process enhanced stars CS 29491–069 and HE 1219–0312

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
² Research School of Astronomy and Astrophysics, Mt. Stromlo Observatory, Cotter Rd., Weston Creek, ACT 2611, Australia
³ Max Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany e-mail: hayek@mpa-garching.mpg.de
⁴ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
⁵ Department of Physics and Astronomy, Uppsala University, Box 515, 75120 Uppsala, Sweden
⁶ Cassiopée, Observatoire de la Côte d'Azur, CNRS, Université de Nice Sophia-Antipolis, Bd. de l'Observatoire, 06300 Nice, France
⁷ Department of Physics and Astronomy, CSCE: Center for the Study of Cosmic Evolution, and JINA: Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, MI 48824, USA
⁸ Department of Astrophysics and Astronomy, University of Chicago, Chicago IL 60637, USA
⁹ Max-Planck-Institut für Chemie (Otto-Hahn-Institut), Joh.-J. Becherweg 27, 55128 Mainz, Germany

Received: 9 October 2008
Accepted: 12 June 2009

Abstract

We report on a detailed abundance analysis of two strongly r-process enhanced, very metal-poor stars newly discovered in the HERES project, CS 29491-069 (, ) and HE 1219-0312 (, ). The analysis is based on high-quality VLT/UVES spectra and MARCS model atmospheres. We detect lines of 15 heavy elements in the spectrum of CS 29491-069, and 18 in HE 1219-0312; in both cases including the Th II 4019 Å line. The heavy-element abundance patterns of these two stars are mostly well-matched to scaled solar residual abundances not formed by the s-process. We also compare the observed pattern with recent high-entropy wind (HEW) calculations, which assume core-collapse supernovae of massive stars as the astrophysical environment for the r-process, and find good agreement for most lanthanides. The abundance ratios of the lighter elements strontium, yttrium, and zirconium, which are presumably not formed by the main r-process, are reproduced well by the model. Radioactive dating for CS 29491-069 with the observed thorium and rare-earth element abundance pairs results in an average age of 9.5 Gyr, when based on solar r-process residuals, and 17.6 Gyr, when using HEW model predictions. Chronometry seems to fail in the case of HE 1219-0312, resulting in a negative age due to its high thorium abundance. HE 1219-0312 could therefore exhibit an overabundance of the heaviest elements, which is sometimes called an “actinide boost”.